It seems in some cases the trailing \0 of the PV is missing (according to Devel::Peek), and in those cases, the regex doesn't match:

use Devel::Peek;
open my $fh, '>', \my $memory_file;

for (qw(abc def ghi jkl mno)) {
    print $fh $_;
    Dump $memory_file;
    if ( $memory_file =~ m/^(.*)$/ ) { 
        print "==> matched: $1\n" 
    }
}

__END__
SV = PV(0x75cce8) at 0x783f70
  REFCNT = 2
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x7b2e10 "abc"\0
  CUR = 3
  LEN = 8
==> matched: abc
SV = PV(0x75cce8) at 0x783f70
  REFCNT = 2
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x7b2e10 "abcdef"\0
  CUR = 6
  LEN = 8
==> matched: abcdef
SV = PV(0x75cce8) at 0x783f70
  REFCNT = 2
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x7b2e10 "abcdefghi"           <--- !!
  CUR = 9
  LEN = 16
SV = PV(0x75cce8) at 0x783f70
  REFCNT = 2
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x7b2e10 "abcdefghijkl"\0
  CUR = 12
  LEN = 16
==> matched: abcdefghijkl
SV = PV(0x75cce8) at 0x783f70
  REFCNT = 2
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x7b2e10 "abcdefghijklmno"\0
  CUR = 15
  LEN = 16
==> matched: abcdefghijklmno
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Update: some further testing (with minor modifications) shows that there are also cases, where it does match despite a missing trailing \0.   So yes, erratic, at best :)